Optical communication systems that have been put into practice so far utilize binary modulation/demodulation techniques with the use of optical intensity. To put it concretely, “0” and “1” of a digital information signal are converted to on-off of the optical intensity of light on a sending side, and fed into an optical fiber, and the light that is propagated along the optical fiber is received on a receiving side and photoelectrically converted to the original digital information signal. In recent years, along with the explosive growth of the Internet, communication capacity required of an optical communication system has been dramatically increased. Measures to cope with the need for vast expansion of the communication capacity by increasing the speed of on-off of the optical intensity, that is, modulation speed, have been taken so far. However, these measures in which it is intended to drastically increase the communication capacity by increasing the modulation speed have the following problem.
There is a problem in that, if the modulation speed is increased, a transmission distance is limited by the chromatic dispersion of an optical fiber. In general, a transmission distance limited by the chromatic dispersion of the optical fiber is shortened in inverse proportion to the square value of the bit rate. In other words, if the bit rate doubles, the transmission distance limited by the chromatic dispersion becomes one fourth. In addition, there is another problem in that, if the modulation speed is increased, a transmission distance is limited by the polarization mode dispersion of the optical fiber. In general, if the bit rate doubles, the transmission distance limited by the polarization mode dispersion is halved. To explain the influence of the chromatic dispersion concretely, if a fiber with a normal chromatic dispersion is used at a bit rate of 10 Gbps, a transmission distance limited by the chromatic dispersion is 60 km, but if the same fiber is used at a bit rate of 40 Gbps, the transmission distance becomes about 4 km. In addition, if the same fiber is used in the next-generation 100 Gbps system, the transmission distance limited by the chromatic dispersion becomes 0.6 km, hence a trunkline optical communication system having a transmission distance about 500 km can not be brought into reality with the use of the same fiber as it is. In order to build an ultrahigh-speed trunkline optical communication system, a special optical fiber that has a negative dispersion characteristic to cancel the chromatic dispersion of a transmission path, a so-called dispersion compensation fiber, is currently installed in transponders and transceivers. This special fiber is expensive, and a sophisticated design is needed to determine the amounts of dispersion compensation fibers respectively installed in the transceivers and the optical transponders, which leads to an advance on the cost of the optical communication system.
Therefore, recently, an optical communication system, which utilizes an OFDM technique as an optical modulation/demodulation technique that can provide large communication capacity, has been researched and developed while getting a lot of attention. The OFDM technique is a technique in which the following processing is performed. First, by respectively setting the amplitudes and phases of a lot of sine-waves that are orthogonal to each other, that is, a lot of sine-waves whose frequencies are integral multiples of the reciprocal of one symbol time (these sine-waves are referred to as subcarriers hereinafter), to be predetermined values in the one symbol time, information is superimposed onto the subcarriers (in other words, the subcarriers are modulated). Next, a carrier is modulated by a signal that bundles the above subcarriers, and the modulated carrier is sent out. This OFDM technique has already put into practice in VDSL (very high bit rate digital subscriber line) systems used for communication between telephone exchange stations and households, power line communication systems used in households, and digital terrestrial television systems. In addition, this OFDM technique is slated to be used for the next-generation mobile network system.
The optical OFDM communication system is a communication system in which the OFDM technique is applied to light that is used as a carrier. In the OFDM technique, a lot of subcarriers are used as described above, and on top of that, multilevel modulation methods such as 4-QAM, 8-PSK, or 16-QAM can be applied to modulation methods for individual subcarriers, hence one symbol time becomes much longer than the reciprocal of the bit rate. As a result, the transmission distance limited by the above-described chromatic dispersion and the polarization mode dispersion becomes much longer than a transmission distance expected in the optical communication system (for example, much longer than 500 km in a domestic trunkline system), hence the above-described dispersion compensation fibers become unnecessary. Therefore, there is a possibility that a low-cost optical communication system is brought into reality.
FIG. 17 is a block diagram of an existing optical OFDM communication system with the use of a direct detection formats.
An optical transmitter 500 and a direct detection optical receiver 600 are connected via an optical fiber 3. When data to be fundamentally communicated are input to the optical transmitter 500 via an input terminal 4, the data are converted to a base-band OFDM signal by a transmission signal processing unit 100 in the optical transmitter 500, and this base-band OFDM signal is amplified by an driver amplifier 10. An optical modulator 501 performs field modulation or intensity modulation on light emitted from a laser 12 with this signal to generate modulated light. This optical OFDM signal reaches the direct detection optical receiver 600 via the optical fiber 3 which is a transmission path. The optical OFDM signal is directly detected and received by a photodiode 201, and converted to an electrical signal. This electrical signal is ideally equal to the above-described base-band OFDM signal, and this signal is amplified by a preamplifier 202, and the amplified signal is converted to digital signals by an AD converter 206. The digital signals output from the AD converter 206 are converted to subcarriers by a reception signal pre-processing unit 220. Subsequently, the subcarriers are demodulated into data to be fundamentally communicated by a reception signal post-processing unit 240, and output from a terminal 5.
FIG. 2 is a functional block diagram of the transmission signal processing unit 100. FIG. 3 is a functional block diagram of the reception signal pre-processing unit 220. FIG. 4 is a functional block diagram of the reception signal post-processing unit 240.
First, data to be transmitted are converted to 2N parallel data by a serial-parallel (S/P) converter 110, where N is the number of subcarriers onto which data are superimposed, and 2N is the number of the parallel data. Although, if the subcarriers are modulated by 4-QAM, the number of the parallel data is 2N, if the subcarriers are modulated by 16-QAM, the number of the parallel data becomes 4N. In other words, serial data are converted to parallel data the number of which is “the bit number of one symbol multiplied by the number of subcarriers”. A subcarrier modulation unit 120 modulates the N subcarriers with these parallel data. These modulated subcarriers are converted to data along a time axis by an inverse FFT (inverse fast Fourier transform) unit 130, and the data are converted to serial data by a parallel-serial (P/S) converter 140. These serial data pass through a digital-analog (D/A) converter 150, and they are sent out to the driver amplifier as analog signals.
At the reception signal pre-processing unit 220, the digitalized electrical reception signals are converted to N parallel data by a serial-parallel (S/P) converter 212. These parallel data are divided into N subcarrier signals at an FFT (fast Fourier transform) unit 213. At the reception signal post-processing unit 240, the data superimposed on the subcarriers are demodulated by a subcarrier demodulation unit 241, and the modulated data are converted to serial data by a parallel-serial (P/S) converter 242, and sent out as information data.
Both in an optical communication system and in a wireless communication system, the fact that a PAPR (peak-to-average power ratio) is large causes a problem. In the case of the wireless communication system, if the linearity of a power amplifier that drives a transmission antenna is inferior, signals are distorted at a peak power, which leads to receiver sensitivity degradation, or interference in adjacent channels owing to the spread of the signal spectra.
In the optical communication system, there is a problem that is caused by a large PAPR and unique to an communication using optical fibers. This problem does not exist in the wireless communication system. It is a phenomenon called a nonlinear phase rotation, in which, at the time when a peak power is large, the phase of light rotates more than the phases of light at other times. This phenomenon occurs owing to the fact that an optical fiber which is a transmission path shows a weak nonlinearity. A nonlinear optical effect, so-called Kerr effect is given by the next expression.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      ϕ          ⁡                      (            t            )                          =                                            ϕ              0                        +                                          ϕ                NL                            ⁡                              (                t                )                                              =                                                    ϕ                0                            +                                                γ                  α                                ·                                  P                  ⁡                                      (                    t                    )                                                                        =                                          ϕ                0                            +                                                γ                  α                                ·                                  P                  ave                                ·                                  PAPR                  ⁡                                      (                    t                    )                                                                                                                      where φ(t) is the instant phase of light; φo is the linear phase; φNL is the nonlinear phase; γ is the non linear constant of optical fiber; α is the loss factor of optical fiber; P (t) is the optical power; Pave is the average optical power; and PAPR(t) is the peak-to-average power ratio (PAPR) at time t. In addition, the alpha-numerals written in italic type in equations will be written in normal type in this specification for convenience. As is clear from Expression 1, the nonlinear phase of the light rotates in proportion to the PAPR.
In a communication system with the use of single wavelength light, the phase rotation of a signal occurs owing to the peak power of the signal itself (this phenomenon is called a self-phase modulation effect), which brings about a waveform distortion owing to a chromatic dispersion, and leads to the increase in the error rate of this communication system. In addition, in a wavelength multiplexing optical communication system, the phase rotation of a signal is induced by the peak powers of signals that have adjacent wavelengths (this phenomenon is called a cross-phase modulation effect), which also leads to the increase in the error rate of this communication system. These phase rotations induce phase rotations of subcarriers of an OFDM signal. To put it more precisely, a random phase rotation in accordance with the PAPR is induced around a constant phase rotation determined by the average power. When this random phase rotation exceeds a threshold for the symbol judgment, the symbol is judged to be an error. For example, in the case where a subcarrier is modulated by QPSK, if the phase rotation of the subcarrier outruns or underruns the ideal symbol point by π/4, the symbol is judged to be an error. Therefore, it is important to use a signal that has the smallest possible PAPR in optical transmission in view of the reduction of the error rate.
In wireless communication systems, many techniques for reducing a PAPR have been proposed. Main techniques are as follows:
(1) A technique in which, while a PAPR is being kept under a certain value using a hard limiter, interferences in the spectra of adjacent wireless channels are controlled using filters; (2) a technique in which, after data are mapped to subcarriers (in other words, after the subcarriers are modulated) plural times, a modulation that provides the smallest PAPR is selected; and (3) a technique in which, a signal is pre-coded, for example, by Trellis coding and given redundancy so that the signal has the small PAPR. Nonpatent literature 1 collectively describes the principles, advantages and drawbacks of these techniques. In addition, as described in nonpatent literature 2, a technique in which the envelope of a wireless signal is kept constant (that is, “PAPR=0 dB” is kept) with the use of phase modulation has been examined recently.
Studies in which the above PAPR reduction techniques are applied to optical OFDM communication systems have already been issued (Refer to nonpatent literatures 3 and 4). In addition, an optical OFDM communication system in which the envelope of a signal is kept constant with the use of the above described phase modulation has already devised according to Japanese Unexamined Patent Application Publication No. 2009-188510 (Patent Literature 1).
In addition, there are some literatures that disclose IQ modulation, direct detection, coherent detection, delay detection, and the like (for example, Patent Literatures (2 to 5).